Production of urea-ammonium sulfate

ABSTRACT

A process for the production of urea-ammonium sulfate granules from urea and ammonium sulfate by several granulation methods. The proportion of ammonium sulfate in the mixture was varied from 15 percent to 70 percent by weight resulting in products containing from 42 percent to 29 percent N and 3 percent to 17 percent S, respectively. Unexpectedly, the urea-ammonium sulfate mixture in these proportions were fluid enough at 250* F. to 325* F. to be prilled and granulated. The presence of the ammonium sulfate increased the strength of the granules significantly.

States Patent [191 Mann, Jr. 1* ,lan. 15, 1974 [5 1 PRODUCTION OFUREA-AMMONIUM 3,050,772 8/1962 von Reppert 71/64 DA x SULFATE 3,110,57211/1963 Yon Reppert 71/64 DA X [75] Inventor: Horace C. Mann, .1112,Florence, Ala. FOREIGN PATENTS 0R APPLICATIONS Assigneez TennesseeValley Authority Muscle 1,535,821 IGelrmany 7l/30 Shoals, Ala. y

[22] Filed: Jan. 27, 1972 Primary ExaminerFrank A. Spear, Jr.

Assistant Examiner-Richard Barnes [21] Appl' 221,262 1 Att0rney-RobertA. Petr usek Related US. Application Data [63] Continuation of Ser. No.155,133, June 21, 1971. [57] ABSTRACT [52] U S Cl 7l/28 7l/6l 71/63 Aprocess for the production of urea-ammonium sul- 7 D A fate granulesfrom urea and ammonium-sulfate by sev- [51] Int Cl C05 9/00 eralgranulation methods. The proportion of ammo- [58] Fi d 63 nium sulfatein the mixture was varied from 15 pere c cent to 70 percent by'weightresulting in products 1 containing from 42 percent-to 29 percent N and 3percent to 17 percent S, respectively. Unexpectedly, the [56] ReferencesCited urea-ammonium sulfate mixture in these proportions UNITED STATESPATENTS were fluid enough at 250". F. to 325 F. to be prilled 3.165.3951/1965 cCamy et al /64 DA X and granulated. The presence of the ammoniumsull/ 2g ii fate increased the strength of the granules signifi- ,1 0ompson et a. 7 1 3,353,949 11/1967 Nau 71/1 X can y 3,030,657 4/1962 vonReppert 71/64 DA X 1 Claim, 3 Drawing Figures BY-PRODUCT DE 4 UREASOLUTION PRAY I SCREENS NQZZLES OVERSIZE v 9 PAN MR GRANULATOR 6 7COOLER PRODUCT 2 34-0-0405 (44 %AMMON1UM SULFATE) 4o-o-o-5s (20%AMMONIUM SULFATE) REcvcLE FINES FLOW DIAGRAM OF GRANULATION PILOT PLANTFOR PRODUCTION OF UREA-AMMONIUM SULFATE PAIENTEUJM 15 mm 3,785,796

SHEU 1 9f 3 OPERABLE RANGE TEMPERATURE RANGE IN WHICH MIXTURE TOOVISCOUS 40o AND FOAMY w TO PRILL O: a g PRILLABLE TEMPERATURE uJ RANGE gPREFERRED 20o RANGE O l l I I o 20 40 so so 100% AMMONIUM SULFATE I00 80so 40 20 o "/0 UREA COMPOSITION Fig. I

PRILLING RANGES OF UREA-AMMONIUM SULFATE MIXTURES IAIDIIEIUIIUsMI3.785796 AMMONIUM SULFATE FEEDE?/ FLUID MIXTURE OF AMMONIUM SULFATE ANDUREA RECYCLED PRILLING MEDIUM SCREEN 1 ENTRIFUGE OFFSIZE TO FEEDERPRODUCT (-6+|2 MESH) Fig, 2 PRILLING UREA-AMMONIUM SULFATE IN LIQUIDMEDIUM 1 PRODUCTION OlEURlEA-AMMONHUM SULFATE This application is acontinuation of my copending application Ser. No. 155,133, filed June21, 1971, for Production of Urea-Ammonium Sulfate.

The invention. herein described may be manufactured and used by or forthe Government for governmental purposes without the payment to me ofany royalty therefor.

My invention relates to a newly developed process for the production ofgranular urea ammonium sulfate; more particularly to the granulation ofa mixture of ammonium sulfate and urea; and still more particularly tothe production of granular urea-ammonium sulfate wherein the resultingproduct is in a highly crystallized form ,of desired and predeterminedparticle size analyzing high in nitrogen fertilizer values andcontaining also therein sulfur values available to the growing plant,which material is useful in liquid and suspension fertilizerpreparations, bulk blending, and for direct application, and whichmaterial provides an economically attractive outlet for theover-abundant ammonium sulfate production capacity prevalent in theindustry.

Most of the ammonium sulfate produced in this country is byproductmaterial from theplastics or steel industries. Until recently up toabout one and a half million tons of this byproduct has been exported.Recently, however, the export market for this material has dropped byapproximately one-third. Present indications are that totalexports ofammonium sulfate will probably decrease further, thus leaving producersof this material with large excesses for use in some alternate productlines or for disposal-at some alternate lower pricing schedule. Tocompound the problem, investigations into the possibilities of alternateroutes to purification of power plant stack gases indicate that possiblyone of the most relatively economical routes will result in the furtherproduction of large amounts of ammonium sulfate liquor. This byproduct,ammonium sulfate, has two drawbacks: (1) its low analysis of plant foodvalues and (2) up to now its particle size when processed into the solidform has been quite variable and oftentimes much too small forsatisfactory use in fertilizer bulk blending operations.

One possibility for enhancing the particle size characteristics of solidammonium'sulfate and at the same time producing a granular high-analysisnitrogen fertilizer containing, in addition to nitrogen values, somesulfur, is by combining such ammonium sulfate with molten urea or anyother suitable molten carrier and prilling the resultant mixture. In,the case where the molten urea is of a concentration of about 99 toabout 99.8 percent, such a process would require no drying step.Although specific details as to the granulation of the mixture ofammonium sulfate and molten urea may be varied as indicated infra, oneembodiment thereof generally follows a procedure somewhat similar tothat shown in Bottai et al, U. S. Pat. 3,578,433, issued May 11, 1971,assigned to the assignee of the present invention. As shown in Bottai etal, the prilling is accomplished in liquid medium usually an oil ofpetroleum source, which procedure eliminates the necessity for largecapital investment for high vertical prilling towers and the like. Theprilling in liquid medium does, however, necessitate a liquid mediumrecovery step for recycle to the process, which liquid medium recoverystep normally entails the use of a'centifuge separator mechanism.

Oil prilling or liquid medium prilling of urea, of course, is not newper se, as is evidenced by U. S. Lettcrs Pat. No. 1,951,518, Mar. 20,1934, Meiser. In fact, Meiser teaches in addition to the oil prilling ofurea the addition of various substances to urea melts to allegedlyreduce the solidification point of what he terms the anhydrous melt,preferably to about 250 F. Meiser includes in his list of additivesammonium sulfate and teaches that no more than 15 percent of thisaddition product should or can be added to the urea and that preferably5 to 10 percent of the addition material is sufficient to accomplish theobserved reduction in melting point.

As indicated above, the preferred embodiment of my process is somewhatsimilar, at least in relation to some of the equipment employed, to theteaching of Bottai et al 433, supra. Another teaching along the lines ofBottai et al is found in application Ser. No. 67,878, filed Aug. 28,1970, Jordan, assigned to the assignee of the present invention. As 'tothe other methods of preparing urea ammonium sulfate mixtures, seeltalian Pat. No. 589,087, Feb. 25, 1959 In another embodiment of myinvention, ammonium sulfate and molten urea is granulated by introducingthe mixture onto a bed of fast-moving cascading particles of undersizeand recycled urea ammonium sulfate contained'on the plate surface of aninclined rotating pan-type granulator. Any suitable pan granulator maybe utilized, preferably the pan is inclined about horizontally androtating somewhere around 2040 r.p.m., depending on the diameter of thepan, it being important only that the pan impart the proper particleaction to the bed thereon so that a portion thereof is slow moving as itbegins to climb the face of the pan and cascading or fast moving in thegeneral region where the pan plate beginsto change from an upward to adownward movement so that to provide for the particles to water fall orcascade away from the rim of the pan into a fan-like or spiraling pathinwardly generally toward the center of the pan, and with sufficientcentri-fugal force provided therein to insure that the falling orcascading particles never approach said center of the pan but do fallaway from the rim thereof. This cascading action and spraying of moltenmaterial onto that fast-moving portion of the pan has been described,for example, in U. S. Pat. No. 3,165,395, McCamy et al. In still anotherembodiment of my invention, l find that I can utilize the TVA drumammoniator-granulator to accomplish granulation of the urea ammoniumsulfate mixture, which drumgranulator is shown in U. S. Pat. No.2,741,545, Nielsson, assigned to the assignee of the present invention.

In developing my process I was, of course, aware that urea solutions ofabout 99.5 percent concentration are produced at a temperature of about300 F. and also that solid urea is melted in commerical equipment atabout this same temperature. However, ammonium sulfate has a relativelyhigh melting temperature, i.e., about 625 F. 1 therefore originallyexpected that mixtures of urea and ammonium sulfate would have to beheated to some temperature intermediate 300 and 600 F. to obtainmixtures of the two materials sufficiently fluid to use in thedroplet-forming device of the liquid medium prilling scheme, and thatthe intermediate temperature required would depend perhaps mostly uponthe proportions of the materials present in the mixtures. However, onsearching the literature, it became obvious from an article in theRussian Journal of cles of ammonium sulfate as long as there issufficient liquid phase, provided primarily by the molten urea, to havea handleable fluid. Thus, although the teachings of Meiser would haveled me to believe that I could not Applied Chemistry, vol. 40, 9,p. 1989(1967) and from 5 handle or granulate a slurry containing more than l5the work done by Meiser, supra, that ammonium sulpercent ammoniumsulfate in molten urea, l have obfate was slightly soluble in urea. TheRussian article served to the contrary that I can incorporate as muchstated that a eutectic point existed at 12l.5 C. and at as about 70percent ammonium sulfate in the urea melt this temperature, a mixture of9 percent ammonium and still produce a granular product by prilling. Thesulfate and 91 percent urea was liquid. Work currently 10 g ular products a matrix 0f th lidifiBd urea done at TVA confirms the Russian datathat a eutectic wherein is embedded the particles of solid ammoniumpoint indeed exists at about this composition and temsulfatey batchtests m de n h l ora ory scal perature'wherein the mixture is completelyliquid. This Showed quite unexpectedly that a mixture containing TVAdata, plus other TVA data at bo h hi h d up to about 70 percent ammoniumsulfate in urea was lower proportions of urea is shown in the tabulationbe- 5 fhhd Slurry the temperature range of f 3000 low. The data athigher proportions of ammonium sul- -i Whlch the Same temPBYamTQYahgewhlch urea f t Shows that as the proportion of ammonium sulfate isnormally held as a melt to be prrlled. I also found that increases, theamount that does not melt also increases. when more thah about 70 P h ofthe ammfhlum Th f it might be expected that the urea o sulfate waspresent, the mixture became too viscous nium sulfate mixture would notbe fluid when solids and foamy flow, even when the temperature was werepresent creased above 325F.

l have found also that when urea and ammonium sulfate are combined inthe manner indicated, the result- Pcrcem Percem Perm quid mg prills werefound to be much stronger than were urea ammonium prills made from 100percent urea. This increase in percent on m strength of the ammoniumsulfate prills over the ammonium strength of 100 percent urea prills isillustrated in Table that does not melt at 1 b l It is therefore anobject of the present invention to I00 0 100 produce well defined prillsor granules of urea ammo- 95 5 100 0 nium sulfate having strength andcrushing characteris- 32 :2 2g g tics greater than straight urea prillsalone and which so 20 9l therefore have excellent handling and storageproperoo 40 s7 s2 50 5s 84 40 44 93 3 5 Another ob ect of the presentinvention is to produce 3g 3g 32 9 well-defined prills or granules ofurea ammonium sull 93 fate having strength and crushing characteristicsA TABLE 1 Strength of Oil-Prilled Urea Ammonium Sulfate Ammonium Prillstrength sulfate in As After Grade prills prilled l week l month 6-8weeks 46-0-0 0 l.5-2 2.5 2.5 2.5 4641-0 025 1-2 2 2.5 2.5 2.545-0-0-0.95 4-s 2.5-3 3-3.5 3 3 430-045 9! 3-4 3-3.5 34.5 34.54|-0-0-4.5 l8-l9 4 4 35-4 3.5-4 35-0-0405 4l-46 3.5-4 3.5-4 3.5 3.5 310-0-|4.5 58-64 3.5 3.5 3.5

Anhydrous and oil free 1 Force in pounds to crush 7 +8 mesh prills Also,Meiser points out in his patent thatu p to 15 percent (but generally5-10 percent is sufficient of an addition compound such as ammoniumsulfate will depress the solidification point of the anhydrous melt.However, Meiser definitely pointed out that no more than a maximum 15percent of an addition compound should be used. The reason for this 15percent maximum value was not explained. Evidently, Meiser found thatthe presence of solids in his melt, when more than about l0 percentammonium sulfate was present, prevented his utilization of the resultantmixture. Contrary to the teachings of Meiser, I have found that I canhandle or distribute a slurry containing undissolved partigreater thanstraight urea prills alone, which therefore have excellent handling andstorage properties, and which contain upwards to as great as percent byweight thereof of ammonium sulfate and preferably from about 20 to 60weight percent as ammonium sulfate.

Still another object of the present invention is to produce well-definedprills of urea ammonium sulfate having strength and crushingcharacteristics greater than straight urea prills alone, which thereforehave excellent handling and storage properties, which contain upwards toas great as 70 percent by weight thereof of ammonium sulfate, andpreferably from about 20 to 60 weight percent as ammonium sulfate, andwhich prills are produced by introducing a mixture of molten urea, saidurea containing at least about 99 percent urea by weight and solidparticulate ammonium sulfate into materials specified by Bottai et al,supra. It should also be noted, as pointed out infra, that myinventionand the process thereof is not restricted to liquid medium prilling,albeit this route has been the one I have found droplet-forming meanswherefrom the droplets pro- 5 the most easy to follow. As noted infra,for existing duced therein comprising a matrix of molten urea and ureaproduction facilities, including already existing a mixture of moltenand solid ammonium sulfate are prilling towers, my mixture of moltenurea and molten quenched, solidifed, and cooled by introduction into andsolid particulate ammonium sulfate can easily be either a liquid mediumimmiscible with said droplets, a adapted to be used therein. pangranulator, or a rotary drum type granulator. 10 FIG. 2 is a flowsheetgenerally illustrating the princi- Still further and more generalobjects and advantages 1 f of my new and 11.0%] P 9 which {65111115 inthe of the present invention will appear from the more dehmqhe harhmehlum Sulfate h havlhg. the tailed description set forth below, itbeing understood, Whe e rhehhohed above' In thleembechmeht, oh however,that this more detailed description is given by Pnhmg hhhzed way ofillustration and explanation only and not by way 3 a howsheet geherahyhlusthahhg the P of limitation since various changes therein may be e ofmy new h hovel proeesewhlehfeshh the made by those skilled in the artwithout departing from hh'que h erhmehlum sulfate R havlhg the hevel thespirit and scope of the present invention. P W$ mehhohee above Inembedhheht pan I granulation scheme is employed.

. My invention, together with further ob ects and ad- Referring now moreparticularly to FIG. 2, most of i there l? F f a the initial work in thepilot plant was carried out with s derat on of the following descriptiontaken n connecsolid urea from a commerical source and a byproduct whhthe accompanying drawmgs m whlch: ammonium sulfate. Both raw materials,plus recycled FIG. l depicts graphically the operable and preferredoffsize urea ammonium sulfate, were heated in a mixranges of prillingmixtures of urea ammonium sulfate e r-melter to ab t 275 to 300 F. andthe mixture of in liquid medium according to the dictates of myinvenmolten urea and molten and solid ammonium sulfate tion. It isunderstood that the liquid medium can be were granulated by oilprilling. Prills containing about most any liquid which is immisciblewith the mixture of percent, 33 percent, and 39 percent nitrogen wereurea and ammonium sulfate and which has the usual produced. Theycontained 61 percent, percent, and desired characteristics for liquidmedium quenching, 30 26 percent ammonium sulfate, respectively, andtheir i.e., low volatility, ease of separation from the prills, sulfurcontent was 14 percent, 10 percent, and 6 peretc. I have found that anumber of liquids can be used, cent by weight, respectively. Detailedoperating data principally oils of hydrocarbon origin and any of the andresults are shown in Table 11 below.

Oil Prilling of Urea Ammonium Sulfate Grade (anhydrous, oil-free)34-0-0-98v '30-0-0-135 Prilling oil 3190 H Mineral seal G Mineral seal GTest No. UAS 2 3 6 5B Melter and Mixing Tank Feed materials" Solid ureaRate, lb./hr.(start-finish) 65.4-70.2 70.8-73.2 79.2-76.8 47.2

Ammonium sulfate Grade 20.2-0-0 20.2-0-0 20.8-0-0 20.8-0-0 14,0 (KarlFischer), by wt. 0.2 0.2 0.2 0.2 Sulfur content, by wt. 23.5 23.5 24.024.0 Rate, lb./hr. (start-finish) 23.9 59.5-59.1 67.2-63.0 78.6

Recycle Grade 35.6-0-0 H 0 (Karl Fischer), by wt. 0.3 Sulfur content, bywt. 9.0 Oil, by wt. 2.5 Screen analysis (Tyler series),

by wt. +10 mesh 41 10 +12 mesh 3 -l2 mesh 56 Rate, lb./hr.(start-finish) 0 0 48.5-86.4 0

Discharge Grade 40.2-0-0 37.4-0-0 30.4-0-0 H O (Karl Fischer, by wt. 0.40.5 0.2

Operating conditions Temperature, F. 300 290 280 280 Retention time,min. 4 3 2 4 Prilling Apparatus Prilling cup Rotational speed, r.p.m.300 350 400 400 Inlet temperature, F. 282 275 270 270 Prilling vesselTemperature F. (start-finish) 105-105 -95 -110 83-85 Trom mel dischargeScreen analysis (Tyler series), by wt.

+6 mesh 4 7 1 6 +10 mesh 71 76 64 71 TABLE 11 Oil Prilling of UreaAmmonium Sulfate Grade (anhydrous, oil-free) 40-0-0-48 34-0-0-9830-0-0-135 Prilling oil 3190 H Mineral seal G Mineral seal Test No. UAS2 3 6 B l0 +12 mesh 7 T '5 s 8 -l2 mesh l8 12 27 14 Oil, by wt. 37 28 2326 Discharge rate (oil-free basis), lb./hr. 89.3-84.1 130.3-132.3194.9226.2 125.8 (start-finish) Prilled Product (6 +12 mesh)Composition, by wt.

Total N 39.0 35.5 33.7 30.2

NH,-N 5.0 9.4 9.1 12.7

Total S 6.2 10.8 10.4 14.7.

Bisulfate-S 0 0.05 0.01 0.0l

11,0 (Karl Fischer) 0.2 0.4 0.3 0.2

Oil 1.6 2.3 2.0 1.4 of prills as ammonium sulfate 26 46 45 61 Bulkdensity, lb./cu. ft. 50 51 51 S6 Apparent density, g./ml. 1.40 l.5l 1.481.58

3190 H oil contained 1/2% lauric acid.

Solid urca (analysis, 1: by wt.: total N, 46.3; 11,0, 0.2, ammoniumsulfate, and recycle fed to mixer-melter.

The urea was air prilled and unconditioned commercial grade.

Ammonium sulfate was a byproduct.

The prilling vessel was inches in diameter with a l-foot-high cylindermounted on top of an lS-inch-high cone with 45 sides.

"Determined from solid urea, ammonium sulfate, and recycle fed tomelter. Products centrifuged at forces of 350, 350, 800, and 600 G,respectively.

In the process solid urea was fed from metering feeder 1 via line 2 tomixer-melter vessel 3 which was equipped with a suitable agitator (notshown) for mixing and a suitable heat source 4 to provide sufficientheat to the process to give a fluid mixture. Solid ammonium sulfate frommetering feeder 5 was fed via line 6 into mixer-melter vessel 3 alongwith recycled oversize and fines, called offsize, from metering feeder 7via line 8. To simulate use of commerical urea solution of 9999.8percent concentration, water from a metering source not shown was fedvia line 9 into the mixermelter vessel 3. The urea-ammonium sulfatemixture from mixer-melter vessel 3 flowed via line 10 to prilling cup11. The prilling cup containedsmall holes and was rotated by a means notshown. As the molten urea and molten and solid ammonium sulfate mixtureflowed through the holes in the prilling cup, small droplets were formedwhich were cooled in liquid medium 12 contained in prilling vessel 13.The cooled droplets of urea ammonium sulfate, hereafter called prills,and a portion of the liquid medium flowed from prilling vessel 13 vialine 14 to trommel l5. Trommel 15 was a device to provide initialseparation of prills from the liquid medium and in this case was acylindrical shaped rotating screen; the excess liquid medium flowedthrough this rotating screen into liquid medium reservoir 16. The prillsand remaining liquid medium flowed via line 17 into centrifuge 18 wheremost of the remaining liquid medium was separated from the urea ammoniumsulfate prills. The liquid medium separated in centrifuge l8 flowed vialine 19 to liquid medium reservoir 16 which contained suitable coolingmeans 20 to remove the heat absorbed from the prills by the liquidmedium. The cooled liquid medium from liquid medium reservoir 16 thenflowed via line 21 through pump 22 back into prilling vessel 13 to coolother droplets of molten urea ammonium sulfate. The prills fromcentrifuge 18 flowed via line 23 into screening device 24 where theprills were separated into onsize (product, oversize, and fines). Theoffsize was returned periodically to metering feeder 7 for reprocessing.The product-sized material from screening device 24 flowed via line 25to a product storage pile (not shown). Liquid medium replacing that lostwith the product could be added to reservoir 16 or to prilling vessel13.

Preliminary tests indicate that the ranges of operating sulfate mixtureprior to prilling. "F Time urea-ammonium sulfate mixture remains atpn'lling temperature. min.... Temperature of prilling medium. "F

Less than 15% ammonium sulfate does not meet a prime object of thisinvention,

to wit, an economically attractive outlet for over'abundant supply ofbyproduct. Also, lesser amounts do not yield sufficient sulfur valuesfor the growing plant in areas deficient therein. Data indicate that themixture began to melt at about 250 F. but the melt was not fluid enoughto prill at 250 F.

Urea may be supplied as either a solid or as a concentrated ureasolution. Also, the ammonium sulfate may be added as a solid or as asolution. Sufficient heat must be provided to the process to (1) heatthe constituents to the prilling temperature, (2) melt the urea (whensolid urea is used) to provide a molten carrier for the solid ammoniumsulfate, (3) melt the urea in the oversize and fines that are recycledto the process, and (4) melt the portion of ammonium sulfate that doesmelt. The raw materials and recycled material could be added to amelter, as illustrated in FIG. 2. Alternatively, the sulfate and recyclefines could be added to a concentrator producing the concentrated ureasolution. Thirdly, hot urea solution, recycle fines, and preheatedammonium sulfate could be mixed in a vessel immediately ahead of theprilling step. No urea ammonium sulfate reaction product was found inthe products. ln the test work thus far, I have used lightweight oils asthe liquid prilling medium. However, many liquid mediums would besatisfactory.

Also the process could be used commercially by air prilling the ureaammonium sulfate mixture in existing conventional urea plants ratherthan oil prilling. For

, this, freshly prepared urea solution (99-99.8 percent I urea, 273300F.) would be fed to a premixing tank where it would be combined withheated ammonium sulfate and recycled offsize material. The generalflowsheet would be similar to FIG. 2 except that the mixermelter vessel3 would likely not have any means of heating (item 4). Other alternatemethods of providing a molten mixture are described in the precedingparagraph. No water (item 9) would be fed to the process. The ammoniumsulfate fed to the process would be preheated to a temperature highenough to give mixtures fluid enough to prill.

Although I do not have facilities for granulating by air prilling, briefexploratory tests have been made to simulate the production of one gradeby this granulation method.

in the prilling tests, fluid mixtures (280-290 F.) of unconditioned ureaand byproduct ammonium sulfate 99 percent -35 mesh.) were poured througha screen which was shaken by hand the the resultant droplets allowed tofall 70 feet through air. Prilling was accomplished by holding thescreens outside an upper window of a tall building and allowing dropletsto fall on a sheet of plastic on the ground. The urea and ammoniumsulfate was proportioned to give mixtures of calculated 34-0-0-1 18grade (47 percent ammonium sulfate and 53 percent urea); however, theammonium sulfate settled very rapidly and even with good mixing somesegregation evidently occurred since the prills recovered contained 37percent nitrogen (calculated to be composed of about 64 percent urea).The very small amount of water required to simulate use of 99.5 percenturea solution was not added.

in an initial test, the air temperature was about 90 F. and no prillswere formed. instead the droplets spatuct nitrogen. Product analysis wasabout 37 percent N and about 9 perc'erit S Production rate was about1000 pounds per hour. Some of the operating results are given in Tablelll below.

TABLE Ill Pan Granulation of Urea Ammonium Sulfate Product grade 0040-0-0 40-0-0 Production rate, tons/hr. 0.5 0.5 0.5 Urea solutions Feedrate, lb./hr. 540 760 780 Concentration, 99.7 99.5 99.5 Temperature, F.297 305 299 Ammonium sulfate Feed rate, lb./hr. 463 195 202 RecycleRate, lb./lb. product 0.5 L6 1.4 Temperature, F. I06 I22 l30 Granulatorproduct Temperature, F. 238 202 223 Screen size (Tyler),

+6 mesh 2l 1 l2 --6 +10 mesh 64 77 82 l0 mesh 15 22 6 tered on hittingthe ground indicating that more height,

cooler air, a countercurrent flow of air, or any desirable combinationof these items would be required for satisfactory granulation by airprilling. in a latter test, made when the air temperature was only about20 F., spattering still occurred when some of the droplets hit theground but some well-shaped prills formed also. The prills were quitehard 3% pounds pressure was required to break prills --7 +8 mesh size.)and of the same strength as those made by prilling in oil. It was notpossible in these exploratory tests, however, to determine the sizedistribution of prills that might be obtained by this method, since alarge proportion of the droplets did not form prills due to therelatively short prilling height and some of the prills formed werecovered by the spattering. The prills collected, however, wereprincipally in the 6- to 12-mesh size range. The results are consideredpromising in regard to the possibility of producing urea ammoniumsulfate in commerical air-prilling equipment such as-normally isemployed for urea.

Urea ammonium sulfate can also be produced in a granular form by feedingconcentrated solutions of urea and ammonium sulfate to any number oftypical devices, such as, for example, rotating pan granulator, rotarydrum-type granulator, Spherodizer, cooled drum, belt flaker, or pugmill.

"' Ammonium sulfate added to urea solution.

With -0.5 pounds of recycle per pound-of product, about 65 percent ofthe granulator product was onsize and about 20 percent was oversize.Granulator product temperature was 238 F.

In two other examples in which the product contained 40 percent N and 5percent S crystalline ammonium sulfate, a byproduct from coke-ovenprocesses in steel mills, represented 20 percent of the feed and waseither added to the stream of recycled, undersize material or wasincorporated in the concentrated urea solution fed through spray nozzlesinto the pan granulator. A recirculation ratio of about 1.5 pounds perpound of product resulted in good granulation.

The products made in these tests were tested physically in comparisonwith straight urea products of commerical producers. Results indicatethat the pilot-plant products containing ammonium sulfate hassubstantially greater strength.

Referring now more specifically to FIG. 3, there is shown a flow-sheetgenerally illustrating the new and novel process using a pan-typegranulator. Streams of concentrated urea solution 1 (produced inequipment not shown but well known to those skilled in the art) andammonium sulfate particles 2 are continuously introduced with arecirculating load of undersize product particles 3 into pan granulatorl, which imparts a rolling and classifying action to the granules beingformed. The stream of ammonium sulfate 2 can be introduced by route A tobe mixed with the recycled undersize stream 3 or by route B to be mixedwith the concentrated urea solution 1. Granular discharge stream 5 iswithdrawn and fed to cooler 6, typically a rotating kilntype cooler,where the hot granules are contacted with air stream 7 to cool thegranules, thus removing the heat imparted to the granules by the streamof urea 11. The stream of cooled granules 8 is then passed to theclassifier 9, typically a set of vibrating screens which separates thestream of granules into an oversize fraction, a product fraction, and anundersize fraction. The stream of oversize granules is 'fed to crusher11; the crushed oversize material 12 is combined with the undersizestream 13, and the resulting stream 3 is recycled to granulator 4. Theproduct-size stream 14 is withdrawn as product.

Granulation in a Spherodizer pugmill, drum granulator, or cooled drum orbelt flaker can be carried out in a similar manner by merelysubstituting the desired type of granulator in place of pan granulator4, as shown in FIG. 3.

While I have shown and described particular embodiments of my invention,modifications and variations thereof will occur to those skilled in theart. I wish it to be understood, therefore, that the appended claims areintended to cover such modifications and variations which are within thetrue scope and spirit of my invention.

What I claim as new and desire to secure by letters patent of the UnitedStates is:

1. A process for the production of strong granules of high analysisurea-ammonium sulfate of easily predetermined closely controlledparticle size range eminently suitable as fertilizer materials whereinsaid ammonium sulfate is evidenced as discrete particles thereofembedded in the matrix of urea, which process comprises the steps of:

l. maintaining in a horizontally inclined rotary pan granulator a bed offines recycled from later mentioned sizing step in continuous andalternately rising and cascading motion;

2. combining and intimately mixing a stream of particulate solidammonium sulfate with a stream of concentrated urea melt in a mixingdevice wherein the temperature in said mixing device is maintained inthe range from about 270 to about 325F and the residence time of thematerial therein is maintained in the range from about 0.1 minutes toabout 10 minutes, said concentrated urea melt containing in the rangefrom about 99 to about 99.8 percent urea by weight and that the feedmaterial constituents are in the range from about 15 to about percent byweight as ammonium sulfate;

3. removing a portion of the resulting free flowing mixture of moltenmelt of urea and substantially particulate ammonium sulfate from saidmixing device and spraying same onto said bed of fines maintained insaid pan granulator;

4. maintaining the temperature of said bed of fines in said pangranulator in the range of about to about 250F;

5. discharging continuously over the lower rim of said pan granulatorthe resulting solid urea-ammonium sulfate granules; and

6. cooling and sizing said withdrawn material and returning theundersize and crushed oversize to the upper rim of said pan granulatorand withdrawing the onsize material as product.

2. combining and intimately mixing a stream of particulate solidammonium sulfate with a stream of concentrated urea melt in a mixingdevice wherein the temperature in said mixing device is maintained inthe range from about 270* to about 325*F and the residence time of thematerial therein is maintained in the range from about 0.1 minutes toabout 10 minutes, said concentrated urea melt containing in the rangefrom about 99 to about 99.8 percent urea by weight and that the feedmaterial constituents are in the range from about 15 to about 70 percentby weight as ammonium sulfate;
 3. removing a portion of the resultingfree flowing mixture of molten melt of urea and substantiallyparticulate ammonium sulfate from said mixing device and spraying sameonto said bed of fines maintained in said pan granulator;
 4. maintainingthe temperature of said bed of fines in said pan granulator in the rangeof about 190* to about 250*F;
 5. discharging continuously over the lowerrim of said pan granulator the resulting solid urea-ammonium sulfategranules; and
 6. cooling and sizing said withdrawn material andreturning the undersize and crushed oversize to the upper rim of saidpan granulator and withdrawing the onsize material as product.